1. Field of the Invention
This invention relates to an improvement in switched current control systems for inductive loads, more particularly, a system for controlling the current in a magnetic bearing.
2. Description of the Prior Art
FIG. 1 illustrates a typical magnetic bearing support system. This configuration has two radial bearings and a single thrust bearing to apply force to the shaft in "x", "y" and "z" axis directions. Magnetic bearings are inherently unstable in open-loop operation, and thus require some type of feedback control to regulate the bearing forces and stabilize the system. The controller relates rotor position to bearing coil current: position measurement is accomplished by shaft displacement sensors located along each control axis. The controller output develops the required current in the bearing coils.
Adaptive, open-loop and multi-axis state space control schemes are examples of techniques which require the flexibility and computational capability of a digital computer for successful implementation. For example, one type of adaptive control automatically adjusts feedback coefficients as the rotor speed is varied. This modifies the dynamic characteristics of the rotor system and adjusts the rotor's critical operating speeds to prevent the system from operating near (or passing through) critical speeds during rotor run-up or steady state operation. MIMO (multiple-input, multiple-output) state-space algorithms which incorporate models of the rotor can also provide significant advantages over simpler control schemes. Such algorithms estimate rotor behavior at locations along the rotor not directly measured by sensors or directly acted upon by a bearing. This information can then be used to exercise control over a section of the rotor not easily accessed by a sensor or a magnetic actuator.
Explicit control of bearing current is accomplished through a power amplifier that drives a current through a load. This current is proportional, over the amplifier bandwidth, to a control signal that is typically a voltage. Since the impedance of a magnetic bearing coil is generally a strong function of the operating conditions, and the output stage of a power amplifier typically applies a voltage to the attached load rather than a current, high-gain current feedback is required to achieve the required true transconductance behavior. In addition, amplifiers used in magnetic bearing applications must possess sufficient voltage overhead to provide adequate current slewing in the typically highly inductive bearing coils, in order to provide bearing force slewing capability. As a result, the required supply voltage will typically be well in excess of the voltage needed merely to provide the coil bias current.
Switching power amplifiers are advantageous for magnetic bearing application because they lower power dissipation by only operating the output transistors in a saturated "on" (low resistance) state or an "off" (very high resistance) state. Power dissipation occurs in these amplifiers primarily while the transistors are switching from one state to the other, with some power also being dissipated in the on state due to the non-zero on-resistance of the transistors. Because most of the power dissipation in these amplifiers occurs during the state transitions, the efficiency relies on keeping the switching rate below some threshold which depends upon the switching characteristics of the output transistors. By switching the output stage at rates in excess of the required amplifier bandwidth (typical switching rates are 10-100 KhZ) and varying the duty cycle of the output waveform, it is possible to create an output signal which combines the desired low frequency component with a higher frequency noise component. A central design issue for a switching transconductance amplifier involves reconciling the requirements of load insensitivity, explicit control of coil current, and efficiency in order to achieve robust, high bandwidth, low distortion operation. Detailed discussion of these issues are presented in .cent.Switching Amplifier Design for Magnetic Bearings," by F. J. Keith et al., Proceedings of the 2nd International Symposium on Magnetic Bearings, Jul. 12-14, 1990, Tokyo, Japan, pp. 211-218.
It is known to include multiple switching components in a switching power amplifier so that the switches effectively reverse the polarity of the control voltage applied to the bearing or other highly inductive load, rather than attempting to throttle the current through the bearing. This arrangement permits current to continue to flow against a strong back electromotive force. This H-Bridge configuration is illustrated in FIG. 2.
In this known configuration, a controller periodically closes both switches in switch pair A while both switches in switch pair B are open, and periodically opens both switches in switch pair A and closes both switches in switch pair B. Varying the periods during which pair A is closed relative to the period during which pair B is closed varies the current through the bearing. The switches are equivalent to resistances that have near zero value when closed and near infinite value when open.
Maintenance of a constant average current in the load using periodic switching of the voltage polarity yields a ripple current in the load in addition to the constant average current as illustrated in FIG. 3. It is often desirable to limit the magnitude of this ripple component. The change in the current as a function of time increases with increased applied voltage. If the circuit resistance and load inductance cannot be changed, then this ripple component can be reduced by either decreasing the applied voltage magnitude or by increasing the switching rate.
Increases in the switching rate are typically limited by the characteristics of the physical switches. The physical switches used are typically solid state devices that exhibit finite transition times between the on and off states. During these finite transition times, the devices present equivalent resistance values that are intermediate to the on and off equivalent resistances. Heat is produced during these transition intervals when current passes through the devices as they offer intermediate resistance to the current flow. This characteristic along with other switching speed limitations of the physical devices constrain the upper limit of switching rate.
For magnetic bearing actuators, the average current desired can change frequently and rapidly. Operation of the magnetic bearing actuator may require that the average current through the actuator remain essentially constant for a relatively long period of time, or very rapid changes in the actuator current may be required. Rapid changes in the current in an inductive load require significant voltage "headroom". Unfortunately, this high voltage magnitude required for rapid changes also increases the current ripple during the intervals when the desired average current is relatively constant.